A water-absorbing resin is recently developed as a substance having high water-absorbing property, and is widely used mainly for a disposable purpose, e.g., as an absorbent body of hygiene product such as a disposable diaper and a sanitary napkin, further, as an agriculture/horticulture water retaining agent, an industrial waterproofing agent, and the like.
Currently, various kinds of water-absorbing resin exist, and a large number of monomers and hydrophilic polymers exist as raw materials for such water-absorbing resins. Among these, a polyacrylic acid (salt)-based water-absorbing resin using acrylic acid and/or salt thereof as a monomer is most abundantly manufactured in the industry due to its high water-absorbing property.
The polyacrylic acid (salt)-based water-absorbing resin is neutralized into polyacrylic acid salt before or after polymerization of acrylic acid, and such a neutralization method and a polymerization method are disclosed in Patent Literatures 1 through 4 and Non Patent Literature 1. Moreover, Non Patent Literature 2 discloses that such acrylic acid encompasses an acrylic acid dimer.
In recent years, mass production of water-absorbing resin has been carried out in accordance with increase in demands for the water-absorbing resin. Under the circumstances, techniques have been developed for stably supplying a large amount of good-quality acrylic acid as a main raw material for the water-absorbing resin.
For example, the following techniques for preventing troubles are known, that is, a technique to remove polymers, which are generated when acrylic acid is transported or stored (Patent Literatures 5 and 6), a technique to transport acrylic acid in an aqueous solution state (Patent Literature 7), and a technique to transport acrylic acid after neutralizing the acrylic acid (Patent Literature 8).
A physical property of a water-absorbing resin is influenced by impurity in acrylic acid. Therefore, in recent years, higher-purity acrylic acid tends to be used as a raw material acid for a water-absorbing resin, in accordance with improvement in performance (i.e., physical property) of water-absorbing resin. Under the circumstances, techniques have been developed for controlling impurity in acrylic acid. Examples of such techniques encompass techniques relating to control of an acrylic acid dimer (Patent Literature 9, Non Patent Literature 1), propionic acid (Patent Literatures 10, 11), acetic acid and propionic acid (Patent Literature 12), allyl acrylate and allyl alcohol (Patent Literature 13), protoanemonin (Patent Literature 14), and furfural (Patent Literature 15).
Further, regarding trace components in the acrylic acid, a technique on a polymerization inhibitor for acrylic acid has been developed. For example, techniques are known which relate to hydroquinone and benzoquinone (Patent Literature 16), methoxyphenols (Patent Literature 17), and an N-oxyl compound and a manganese compound (Patent Literature 18).
Furthermore, techniques are known such as a technique to use acrylic acid which has been manufactured from a sustainable raw material (Patent Literature 19), a technique to use acrylic acids purchased from two or more manufacturers (Patent Literature 20), and a technique relating to countries manufacturing acrylic acid (Patent Literature 21).
Despite the improved techniques above cited, there is still room for improvement in mass production of a water-absorbing resin from acrylic acid. For example, in general, a lower impurity content in acrylic acid is considered to be better. From this, the water-absorbing resin tends to be obtained from acrylic acid that contains impurity as little as possible, as disclosed in Patent Literatures 11 through 14, etc.
However, in order to reduce the impurity content, purification of acrylic acid is carried out with a large amount of labor and cost, and this causes a problem that expansion of scale in producing water-absorbing resin from high-purity acrylic acid is greatly restricted from the viewpoint of cost, procurement of acrylic acid, and the like.
In a case where propionic acid, which is one of impurities, is controlled to be contained by a predetermined amount or more in polymerization, a water absorption capacity (CRC) of the water-absorbing resin is improved (Patent Literatures 10 and 11). However, if a large amount of the propionic acid remains after the polymerization, a resultant water-absorbing resin sometimes emits stronger unpleasant odor (acid odor).